1. Field of the Invention
The present invention relates to a method and related communication device in a wireless communication system for improving a semi-persistent scheduling (SPS) resource release process, and more particularly, to a method and related communication device for improving the reliability of acknowledgement responding to a SPS resource release command.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs). The radio protocol stacks of E-UTRAN is given including a radio resource control layer (RRC), a packet data convergence protocol layer (PDCP), a radio link control layer (RLC), a media access control layer (MAC), and a physical layer (PHY).
A dynamic scheduling function is used in MAC for utilizing shared channel (SCH) resources. When sharing resources between UEs, MAC in eNB dynamically allocates physical layer resources for the downlink (DL)-SCH and uplink (UL)-SCH depends on the traffic volume, the Quality of Service (QoS) requirements of each UE, and associated radio bearers. In order to utilize SCH resources more efficiently, a semi-persistent scheduling (SPS) function is introduced in the LTE system and is used in MAC, for serving upper layer applications which generates semi-static size data periodically, e.g. VoIP services. The eNB activates SPS resource by sending a physical downlink control channel (PDCCH) signaling. According to current specification, a UE reply for SPS resource activation is not necessary.
There are two error cases in SPS resource activation. One is false activation that the eNB activates SPS resource on a UE which is not a target UE, which may result in SPS resources interference when the same SPS resource is also activated on the target UE. Current specification defines virtual cyclic redundancy check (CRC) in PDCCH to prevent SPS resources interference caused by the harmful false activation. The other is failure activation that the eNB does not activate SPS resource on the target UE successfully. It is not a considerable concern since there is no potential interference between UEs. The eNB can find out the failure activation at the coming SPS resource and activate SPS resource again.
Similar to SPS resource activation, SPS resource release is also done by an explicit PDCCH signaling including an SPS cell radio network temporary identifier (C-RNTI). According to previous specification, the eNB does not require waiting for a UE reply for SPS resource release. It is a different story for error cases of SPS resource release, however. One is false release that the eNB releases SPS resource on the UE by mistake, which does not result in harmful SPS resources interference and the eNB can activate SPS resource on the UE again if the eNB likes to do so. The other is failure release that the eNB transmits an SPS resource release command to the UE while the UE fails to receive the SPS resource release command and still uses SPS resources previously assigned. If the eNB reassigns SPS resources prematurely to another UE without waiting for any confirmation responding to the SPS resource release command from the UE, SPS resources interference between UEs may happen. For UL SPS, the UE keeps transmitting on revoked SPS resources and interferes with the transmission of another UE which currently owns SPS resources. For DL SPS, the UE keeps receiving on revoked SPS resources but cannot decode transport blocks correctly since the data is for another UE, and as a result, a corresponding negative-acknowledgement (NACK) transmitted by the UE probably collides with the ACK/NACK of the current owner.
Duo to poor reliability of the PDCCH signaling, the eNB is not confident that the UE indeed receives the PDCCH signaling of SPS resource release, and SPS resources interference caused by failure SPS resource release may happen. In order to prevent the potential interference, for UL case, the eNB reuses SPS resources after waiting and seeing nothing sent from the UE on the next UL SPS resource; and for DL case, the eNB reuses SPS resources after waiting and seeing no corresponding ACK/NACK sent from the UE with respect to the next DL SPS resource. The above solution is also called implicit release, which costs the eNB extra time on waiting.
For improving SPS resource release, the 3GPP further defines that the UE has to confirm DL SPS resource release by replying a physical uplink control channel (PUCCH) ACK, which is more efficient than the eNB waiting and seeing if no data on the next SPS resource. The eNB sends an SPS resource release PDCCH signaling at subframe n which implicitly defined PUCCH resource at subframe (n+4) for the UE to response a PUCCH ACK if the release is successful, therefore, the eNB can safely reassign SPS resource to another UE. However, the PUCCH ACK is error-prone and its reliability is around 10−1˜10−2. Presently, the PUCCH ACK reply is only for DL SPS resource release. Whether the eNB can reuse SPS resources directly after receiving the PUCCH ACK is questionable.